1. Field of the Invention
The present invention relates to an improved catalyst for steam reforming processes such as tubular reforming, heat exchange reforming, catalytic partial oxidation (CPO), autothermal reforming and secondary reforming. The invention concerns also a method of preparation of the catalyst and process for reforming using the catalyst. More particularly, the invention relates to a reforming catalyst for use in autothermal reforming (ATR) or catalytic partial oxidation (CPO) processes.
2. Description of the Related Art
As used herein, autothermal reforming (ATR) encompasses all variations of this technology, including air and oxygen fired secondary reforming. The term secondary reforming is normally used when the resulting synthesis gas is used as ammonia synthesis gas. The present invention mainly focuses on oxygen-blown autothermal reforming. However, what it is presented here is of use also for air-blown autothermal reforming and catalytic partial oxidation (CPO). Typically, air-blown autothermal reforming is used in ammonia plants and in general the operating conditions of the autothermal reformer are less demanding because of the moderating effect of nitrogen in the air and higher steam-to-carbon ratios.
The production of synthesis gas from natural gas, oil, coal, coke, naphta and other carbonaceous resources is typically carried out via steam reforming, autothermal reforming, catalytic partial oxidation or gasification reactions. The synthesis gas (syngas) contains hydrogen, carbon monoxide, carbon dioxide and water as the major components.
The use of autothermal reforming technology for the treatment of process gas which has partially been reformed upstream is well established. The partially reformed gas results normally from the treatment of hydrocarbon feeds which have been passed through tubular reformers or heat exchange reformers. Natural gas feeds may also be directly passed through the autothermal reformer, optionally after the feed gas has passed through an adiabatic prereformer.
In an autothermal reformer (ATR), pre-heated hydrocarbon feedstock is subjected to exothermic internal combustion with oxygen, i.e. partial oxidation, followed by endothermic steam reforming of the partially oxidized feedstock in a fixed bed of catalyst. The chemical reactions within this type of reformer are combinations of combustion and steam reforming reactions. The ATR consists roughly of a refractory-lined pressure vessel, a combustion chamber and a fixed bed of catalyst. A burner mounted on top of the reactor provides for the mixing of a pre-heated hydrocarbon feed stream, such as a methane-rich stream, together with an oxygen containing stream, such as air or mixture of oxygen/steam. Oxygen may be supplied in substoichiometric amounts (less than it is required for full combustion of the hydrocarbon feed) and flame ignition reactions of the hydrocarbon feed take place in the combustion chamber located in the upper portion of the reactor. The combustion chamber is defined by the region in between the burner at the top of the reactor and the fixed bed(s) of catalyst and may also comprise a region where further conversion of the hydrocarbon feed occurs due to homogeneous gas-phase reactions. The final hydrocarbon conversion takes place by heterogeneous catalysis in one or more fixed beds of suitable catalyst arranged in the lower portion of the reactor.
The flame ignition reactions representing the partial oxidation of hydrocarbon feedstock are highly exothermic, while the final hydrocarbon conversion in the fixed bed of catalyst is endothermic and is conducted in the presence of for example steam. The exothermic reactions provide for the heat necessary for the endothermic catalytic steam reforming. In an autothermal reformer typical temperatures of the process gas leaving the combustion chamber are in the range 800-1600° C., more specifically in the range 900-1400° C. The gas cools by means of the endothermic steam reforming reaction in the catalyst bed to 850-1100° C. In the region above the fixed bed of catalyst peak flame temperatures of 2000-3500° C. may be achieved. The actual temperatures may vary depending on for instance whether the reactor is air-blown or oxygen-blown.
Steam reformers such as tubular reformers, heat exchange reformers, catalytic partial oxidation (CPO) reformers and particularly autothermal reformers are normally operated with nickel based catalysts (nickel as the only metal) of determined shapes such as ring shaped catalysts. Unfortunately, because of the severe conditions prevailing in these reformers, particularly in autothermal reformers, we have observed that there may be nickel depletion at the geometrical surface of the catalyst bodies as well as nickel sintering and thereby loss of effective catalytic surface area. Overall the catalyst looses stability and activity over time.
More specifically, problems related to particularly ATR operation include vaporization of nickel and rapid nickel sintering. Nickel volatilization seems to be the result of nickel particles from the reforming catalyst reacting with steam according to Ni(s)+H2O═Ni(OH)2 (g).
In addition, the autothermal reformer is refractory-lined, and the catalyst bed is protected by a layer of refractory tiles. A key aspect is to maintain a low pressure drop across the catalyst bed to eliminate the risk of gas bypass into the refractory lining leading to hot spots on the reactor shell. The refractory materials are based on alumina and small amounts of alumina evaporate from these materials at the high temperature in the combustion chamber according to Al2O3(s)+2H2O (g)=2 Al(OH)3 (g). This alumina vapour then condenses (or solidifies or deposits) on the catalyst which is kept relatively cool by the endothermic reforming reaction. As a result there is a gradual lowering of the void of the catalyst that leads to increasing pressure drop over the catalyst bed.
US 2005/0089464 discloses a catalyst for partial oxidation based on Rh on alumina and a catalyst for steam reforming based on Ni on alumina. The metal loading is high, i.e. in the range 5-30 wt %.
U.S. Pat. No. 7,230,035 discloses a catalyst provided with a pore blocking layer between the support and catalytic layer by which more than 60% of the active material is in the outer shell, i.e. shows an egg-shell profile. The catalytic active material can be iridium, rhenium or rhodium.
US 2009/0108238 discloses a catalyst for reforming hydrocarbons comprising metals such as platinum, palladium, rhodium, iridium, ruthenium deposited on a support produced from a mixture of low surface area material and high surface area material.
EP-A-1338335 discloses catalysts for hydrocarbon reforming including Ir and Co, or Rh and Co, or Ru and Co, on a support of ceria and alumina. The weight content of Ir or Rh or Ru is about the same as Co. This citation is silent about the use of egg-shell catalysts.
US2007/0238610 discloses fuel reformer catalysts applied as wash coats in foams and monoliths for fuel cell applications. The disclosed catalysts include dual stage catalysts such as 2 wt % Ir-2 wt % Ni on La2O3 catalyst followed by a catalyst containing Pd or Pt such as 1 wt % Pd-5 wt % Ni. The dual stage catalysts provide higher hydrogen generation than their single-stage counterparts. Where Rh is used, the catalyst consists of Rh, Pt or Pd, and Ni. The addition of Rh is said to improve the catalyst resistance to sulphur poisoning and coke formation. This citation is silent about the use of egg-shell catalysts.
WO-A-9737929 discloses a experimental reactor for conducting partial oxidation reactions involving the use of monoliths with catalyst systems involving the use of Rh in the first catalyst bed and Ru in the second bed, alternatively Rh in the first bed and Ni in the second bed. There is no disclosure of a Rh—Ni or Ir—Ni catalyst system nor the use of egg-shell catalysts.
WO-A-2010078035 describes briefly and broadly the use of Ni—Ir catalysts in ATR applications, in particular a Ni—Ir catalyst with 2.5 wt % Ni and 0.5 wt % Ir with about 0.25 wt % variation for optimization. This citation is silent about the use of egg-shell catalysts.
WO-A-2007/015620 discloses the use of Ru-supported Ni-based catalysts for steam reforming as well as Ir-supported Ni-based catalysts, the former exhibiting superior steam reforming activity. The catalyst is prepared in the form of a powder for which full impregnation of Ru or Ir throughout the particles is expected to be obtained. The citation is thus silent about the use of egg-shell catalysts.
US2008/0265212 discloses sulphur tolerant catalysts for production of synthesis gas and hydrogen via steam reforming at about 500° C. for fuel cell applications. The catalysts are in the form of powders and include Rh—Ni on i.a. ceria-alumina. Since the catalyst is in the form of powder, full penetration of the active metals within the particles is expected. Accordingly, this citation is silent about egg-shell catalysts.
U.S. Pat. No. 5,616,154 discloses broadly the use of Rh and Ru as catalysts on several supports including alumina for converting liquid organic materials at low temperatures and high pressure (300-450° C., above 130 atm) into gas containing methane, carbon dioxide and hydrogen, i.e. methanation. The metals Ir, Pt, and Pd optionally in combination with reduced Ni as second catalyst are also mentioned as a possibility for effectively conducting CO-methanation. Of the tested catalysts none are bimetallic Ir—Ni or Rh—Ni and the methanation process concerned is a completely different field of use than in the present invention.
US 2008/0197323 discloses the use of catalysts in for instance autothermal reforming, where catalytic activity on a first (top) layer is enhanced by using in this layer catalysts with higher geometrical surface area (GSA) than in subsequent layers. The active metal in the catalyst is nickel, which may be replaced by metals including platinum, palladium, iridium, ruthenium and rhodium.
In Nitrogen and Syngas 2010 International Conference, Bahrain Feb. 28-Mar. 3, 2010, p. 97-109, it is broadly suggested to provide the top of a catalyst bed in an autothermal reformer with catalyst bodies having a low GSA in which said bodies have more than one through hole while the lower and major part of the catalyst bed is provided with smaller catalyst bodies having a higher GSA and also containing more than one through hole.
EP-A-0625481 describes a process for high temperature reforming, for instance autothermal reforming, in which the catalyst bed comprises an upper and a lower layer, with the catalysts in the upper layer having reduced activity. The reduced activity is said to be possible by increasing the particle size of the catalyst bodies in the upper layer.